remote sensing Article Mapping Hydrothermal Zoning Pattern of Porphyry Cu Deposit Using Absorption Feature Parameters Calculated from ASTER Data Mengjuan Wu 1,2,3,4, Kefa Zhou 1,2,3,4, Quan Wang 5,6,* and Jinlin Wang 1,2,3,4 1 State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China 2 Xinjiang Key Laboratory of Mineral Resources and Digital Geology, Chinese Academy of Sciences, Urumqi 830011, China 3 Xinjiang Research Centre for Mineral Resources, Chinese Academy of Sciences, Urumqi 830011, China 4 University of Chinese Academy of Sciences, Beijing 100049, China 5 Faculty of Agriculture, Shizuoka University, Shizuoka 422-8529, Japan 6 Research Institute of Green Science and Technology, Shizuoka University, Shizuoka 422-8529, Japan * Correspondence: [email protected]; Tel.: +81-54-2383683 Received: 3 June 2019; Accepted: 18 July 2019; Published: 22 July 2019 Abstract: Identifying hydrothermal zoning pattern associated with porphyry copper deposit is important for indicating its economic potential. Traditional approaches like systematic sampling and conventional geological mapping are time-consuming and labor extensive, and with limitations for providing small scale information. Recent developments suggest that remote sensing is a powerful tool for mapping and interpreting the spatial pattern of porphyry Cu deposit. In this study, we integrated in situ spectral measurement taken at the Yudai copper deposit in the Kalatag district, northwestern China, information obtained by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), as well as the spectra of samples (hand-specimen) measured using an Analytical Spectral Device (ASD) FieldSpec4 high-resolution spectrometer in laboratory, to map the hydrothermal zoning pattern of the copper deposit. Results proved that the common statistical approaches, such as relative band depth and Principle Component Analysis (PCA), were unable to identify the pattern accurately. To address the difficulty, we introduced a curve-fitting technique for ASTER shortwave infrared data to simulate Al(OH)-bearing, Fe/Mg(OH)-bearing, and carbonate minerals absorption features, respectively. The results indicate that the absorption feature parameters can effectively locate the ore body inside the research region, suggesting the absorption feature parameters have great potentials to delineate hydrothermal zoning pattern of porphyry Cu deposit. We foresee the method being widely used in the future. Keywords: porphyry Cu deposit; hydrothermal zoning pattern; absorption feature parameters; ASTER 1. Introduction Porphyry Cu systems are defined as large volumes (10->100 km3) of hydrothermally altered rock centered on porphyry Cu stocks, presently supplying nearly three quarters of the world’s Cu, half the Mo, perhaps one fifth of the Au, most of the Re, and minor amounts of other metals (silver, palladium, tellurium, selenium, bismuth, zinc, and lead) [1]. Porphyry Cu deposits typically occur in association with hydrothermal alteration zoning patterns that, in general, comprise potassic, phyllic, and propylitic from the center to the outward [2], deploying recognizable spatial features. The mineralization is closely related to alteration, which is a sign of mineralization scale and enrichment degree of the ore body. Furthermore, it is well known [1,2] that the ore body is mainly distributed in quartz-phyllitization Remote Sens. 2019, 11, 1729; doi:10.3390/rs11141729 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 1729 2 of 19 or potassic alteration zone, the larger the size of the deposit, the stronger the alteration. Moreover, the better the zoning, the higher the mineralization enrichment [3]. Consequently, a clear understanding of the characteristics and spatial distribution pattern of hydrothermal alteration zoning pattern in porphyry Cu systems plays a critical role in locating a prospecting target. The traditional method for identifying the alteration zone is based on the restoration of the original rock properties and is determined according to the type and intensity of the altered minerals and the contents of major elements (e.g., SiO2,K2O, Na2O, CaO, FeO, Fe2O3, and so on), which is very time-consuming and labor extensive. The Yudai deposit, which is the research site of this study, detailed drill core logging, and petrographic study have been used to delineate the alteration/mineralization zones, and the whole-rock geochemical data for the Yudai quartz diorite porphyry have been studied to discuss petrogenesis [4]. Comparatively, a recently developed method of using statistics to process geochemical profile data provides relatively quickly and accurately delineate the alteration-mineralization range and zoning of porphyry body and surrounding rock for narrowing the prospecting target area [3]. However, such geologically based approaches, in general, require a large number of hand specimen and thin section identification, making the division of the alteration zone a more complicated task and hard to keep up with the needs of prospecting and evaluation. On the other hand, the mineral assemblages in alteration zone deploy diagnostic spectral absorption features in the visible near-infrared and shortwave infrared wavelength regions, such as phyllosilicates, carbonates, sulfates, Mg, Fe-bearing, and OH-bearing minerals [5–9]. Accordingly, remote sensing data with sufficient spatial and spectral information are promising for identifying spatial distributions of mineral groups. Using multispectral data to extract alteration information in the exposed area of bedrock has already achieved good results in previous studies [10–13]. To date, most popularly applied optical satellite-borne data on identifying alteration zone are Landsat Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), Advanced Land Imager (ALI), and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, due preliminary to their relatively high spatial resolution and affordable cost. For example, Landsat Thematic Mapper/Enhanced Thematic Mapper+ (TM/ETM+) images have been used as a tool to identify hydroxyl-bearing minerals in the reconnaissance stages of porphyry copper/gold exploration [14,15]. While the Advanced Land Imager has six unique wavelength channels spanning the visible and near-infrared (400–1000 nm), which is especially useful for detecting iron minerals in porphyry copper deposits [16,17]. However, although the hydroxyl-bearing alteration minerals have been separated from unaltered surrounding rocks in TM/ETM+ image, the individual alteration minerals cannot be identified due to the limitation of spectral resolution in shortwave infrared (SWIR). In comparison, ASTER data has a relatively narrow spectral resolution in the shortwave infrared region, which can facilitate highlighting the presence of spectral absorption characteristics of Al-O-H, Mg-O-H, Fe-O-H, Si-O-H, and CO3 molecular bonds, and would make it superior to other multispectral data (including TM) in the absence of usable hyperspectral data. In detail, illite/muscovite, which is a representative alteration mineral in the phyllic zone, yields an intense Al-OH absorption feature centered at 2200 nm, coinciding with ASTER band 6 (2185–2225 nm). The mineral assemblages of the outer propylitic zone, including epidote, chlorite, and calcite, exhibit absorption features situated at 2350 nm, which coincide with ASTER band 8 (2295–2365 nm). As thus, ASTER data have been widely used for geological/structural mapping and ore minerals exploration, particularly for porphyry copper deposits [18–23]. Most popular methods of applying ASTER data on mapping the hydrothermal zoning pattern of porphyry Cu deposit include Principle Component Analysis (PCA), Band Ratios (BR), Spectral Angle Mapper (SAM), and Mixture-Tuned Matched-Filtering (MTMF) [20]. Most of these methods have been used for identifying individual alteration mineral, but seldom in determining the absolute contents (intensity) of individual minerals, a sign of mineralization enrichment, and ore body position. Furthermore, the mapped hydrothermal zoning patterns using those methods can be misleading when the representative alteration minerals are distributed in different alteration zones. RemoteRemote Sens. Sens.2019 2019, 11, 11, 1729, x FOR PEER REVIEW 3 of3 of19 19 position. 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